Preparation of rare earth permanent magnet material

ABSTRACT

A method for preparing a rare earth permanent magnet material comprises the steps of: disposing a powder comprising one or more members selected from an oxide of R 2 , a fluoride of R 3 , and an oxyfluoride of R 4  wherein R 2 , R 3  and R 4  each are one or more elements selected from among rare earth elements inclusive of Y and Sc on a sintered magnet form of a R 1 —Fe—B composition wherein R 1  is one or more elements selected from among rare earth elements inclusive of Y and Sc, and then heat treating the magnet form and the powder at a temperature equal to or below the sintering temperature of the magnet in vacuum or in an inert gas. The result high performance, compact or thin permanent magnet has a high remanence and coercivity at a high productivity.

INCORPORATED-BY-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.10/572,753, filed Mar. 21, 2006, which is a National Stage Entry ofPCT/JP05/05134 and claims the benefit of priority from Japanese PatentApplication No. 2004-304543, filed on Oct. 19, 2004 and Japanese patentApplication No. 2004-377379, filed on Dec. 27, 2004, the entire contentsof which being incorporated herein by reference.

TECHNICAL FIELD

This invention relates to a method for preparing a R—Fe—B systempermanent magnet in which the coercive force of a sintered magnet formis enhanced while controlling a decline of its remanence; and moreparticularly, to a method for preparing a high performance, compact orthin rare earth permanent magnet material.

BACKGROUND ART

Nd—Fe—B system permanent magnets have a growing range of application dueto their excellent magnetic properties. While electronic equipmenthaving magnets built therein including computer-related equipment, harddisk drives, CD players, DVD players, and mobile phones are currentlyunder the trend toward size and weight reductions, higher performanceand energy saving, there exists a demand to enhance the performance ofNd—Fe—B magnets, especially compact or thin Nd—Fe—B sintered magnets.

Indexes for the performance of magnets include remanence (or residualmagnetic flux density) and coercive force. An increase in the remanenceof Nd—Fe—B sintered magnets can be achieved by increasing the volumefactor of Nd₂Fe₁₄B compound and improving the crystal orientation. Tothis end, a number of modifications have been made on the process. Withrespect to the increased coercive force, among different approachesincluding grain refinement, the use of alloy compositions with greaterNd contents, and the addition of effective elements, the currently mostcommon approach is to use alloy compositions having Dy or Tb substitutedfor part of Nd. Substituting these elements for Nd in the Nd₂Fe₁₄Bcompound increases both the anisotropic magnetic field and the coerciveforce of the compound. The substitution with Dy or Tb, on the otherhand, reduces the saturation magnetic polarization of the compound.Therefore, as long as the above approach is taken to increase coerciveforce, a loss of remanence is unavoidable.

In Nd—Fe—B magnets, the coercive force is given by the magnitude of anexternal magnetic field which creates nuclei of reverse magnetic domainsat grain boundaries. Formation of nuclei of reverse magnetic domains islargely dictated by the structure of the grain boundary in such a mannerthat any disorder of grain structure in proximity to the boundaryinvites a disturbance of magnetic structure, helping formation ofreverse magnetic domains. It is generally believed that a magneticstructure extending from the grain boundary to a depth of about 5 nmcontributes to an increase of coercive force (See non-patent reference1). The inventors found that by concentrating trace Dy or Tb only inproximity to the grain boundaries to increase the anisotropic magneticfield only in proximity to the boundaries, the coercive force can beincreased while suppressing significant decline of remanence (see patentreference 1). Subsequently, the inventors established a productionmethod comprising separately preparing a Nd₂Fe₁O compound compositionalloy and a Dy or Tb-rich alloy, mixing them and sintering the mixture(see patent reference 2). In this method, the Dy or Tb-rich alloybecomes a liquid phase during the sintering and is distributed so as tosurround the Nd₂Fe₁₄B compound. As a consequence, substitution of Dy orTb for Nd occurs only in proximity to grain boundaries in the compound,so that the coercive force can be effectively increased whilesuppressing significant decline of remanence.

However, since the two types of alloy fine powders in the mixed stateare sintered at a temperature as high as 1,000 to 1,100° C., theabove-described method has a likelihood that Dy or Tb diffuses not onlyto the boundaries, but also into the interior of Nd₂Fe₁₄B grains. Anobservation of the microstructure of an actually produced magnet showsthat Dy or Tb has diffused to a depth of about 1 to 2 μm from theboundary in a grain boundary surface layer, the diffused area reaching60% or more when calculated as volume fraction. As the distance ofdiffusion into grains becomes longer, the concentration of Dy or Tb nearthe boundaries becomes lower. To positively suppress the excessivediffusion into grains, lowering the sintering temperature may beeffective, but this measure cannot be practically acceptable because itcompromises densification by sintering. An alternative method ofsintering at lower temperatures while applying stresses by means of ahot press or the like enables densification, but poses the problem ofextremely reduced productivity.

On the other hand, it is reported for small magnets that coercive forcecan be increased by applying Dy or Tb on the magnet surface bysputtering, and heat treating the magnet at a temperature lower than thesintering temperature, thereby causing Dy or Tb to diffuse only to grainboundaries (see non-patent references 2 and 3). This method allows formore effective concentration of Dy or Tb at the grain boundary andsucceeds in increasing the coercive force without a substantial loss ofremanence. As the magnet becomes larger in specific surface area, thatis, the magnet form becomes smaller, the amount of Dy or Tb fed becomeslarger, indicating that this method is applicable to only compact orthin magnets. However, there is still left the problem of poorproductivity associated with the deposition of metal coating bysputtering or the like.

Patent reference 1: JP-B 5-31807

Patent reference 2: JP-A 5-21218

Non-patent reference 1: K. D. Durst and H. Kronmuller, “THE COERCIVEFIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism andMagnetic Materials, 68 (1987), 63-75

Non-patent reference 2: K. T. Park, K. Hiraga and M. Sagawa, “Effect ofMetal-Coating and Consecutive Heat Treatment on Coercivity of ThinNd—Fe—B Sintered Magnets,” Proceedings of the Sixteen InternationalWorkshop on Rare Earth Magnets and Their Applications, Sendai, p. 257(2000)

Non-patent reference 3: K. Machida, H. Kawasaki, M. Ito and T. Horikawa,“Grain Boundary Tailoring of Nd—Fe—B Sintered Magnets and Their MagneticProperties,” Proceedings of the 2004 Spring Meeting of the Powder &Powder Metallurgy Society, p. 202

SUMMARY OF THE INVENTION Problem to Be Solved by the Invention

An object of the invention, which is made to solve the foregoingproblems, is to provide a method for preparing a R—Fe—B sintered magnethaving a high coercive force at a high productivity wherein R is one ormore elements selected from among rare earth elements inclusive of Y andSc.

Means for Solving the Problem

The inventors have discovered that when a R¹—Fe—B sintered magnet,typically a Nd—Fe—B sintered magnet is heated with a powder comprisingone or more members selected from an oxide of R², a fluoride of R³, andan oxyfluoride of R⁴ wherein R², R³ and R⁴ each are one or more elementsselected from among rare earth elements inclusive of Y and Sc beingpresent in the magnet surface, R², R³ or R⁴ contained in the powder isabsorbed in the magnet form whereby the coercive force is increasedwhile significantly suppressing a decline of remanence. Particularlywhen a fluoride of R³ or an oxyfluoride of R⁴ is used, R³ or R⁴ isefficiently absorbed in the magnet along with fluorine, resulting in asintered magnet having a high remanence and a high coercive force. Thepresent invention is predicated on this discovery.

The present invention provides a method for preparing a rare earthpermanent magnet material as defined below.

First Embodiment

A method for preparing a rare earth permanent magnet material comprisingthe steps of:

-   -   disposing a powder comprising one or more members selected from        an oxide of R², a fluoride of R³, and an oxyfluoride of R⁴        wherein R2, R³ and R⁴ each are one or more elements selected        from among rare earth elements inclusive of Y and Sc on a        sintered magnet form of a R¹—Fe—B composition wherein R¹ is one        or more elements selected from among rare earth elements        inclusive of Y and Sc, and    -   heat treating the magnet form and the powder at a temperature        equal to or below the sintering temperature of the magnet in        vacuum or in an inert gas.

Second Embodiment

A method for preparing a rare earth permanent magnet material accordingto the first embodiment, wherein the sintered magnet form to be heattreated has a shape having a dimension of up to 100 mm along its maximumside and a dimension of up to 10 mm in a magnetic anisotropy direction.

Third Embodiment

A method for preparing a rare earth permanent magnet material accordingto the second embodiment, wherein the sintered magnet form to be heattreated has a shape having a dimension of up to 20 mm along its maximumside and a dimension of up to 2 mm in a magnetic anisotropy direction.

Fourth Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first three embodiments, wherein the powder comprising oneor more members selected from an oxide of R², a fluoride of R³, and anoxyfluoride of R⁴ is present in a magnet-surrounding space within adistance of 1 mm from the surface of the magnet form and at an averagefilling factor of at least 10%.

Fifth Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first four embodiments, wherein the powder comprising oneor more members selected from an oxide of R², a fluoride of R³, and anoxyfluoride of R⁴ has an average particle size of up to 100 μm.

Sixth Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first five embodiments, wherein in said one or moremembers selected from an oxide of R², a fluoride of R³, and anoxyfluoride of R⁴ wherein R², R³ and R⁴ each are one or more elementsselected from among rare earth elements inclusive of Y and Sc, R², R³ orR⁴ contains at least 10 atom % of Dy and/or Tb.

Seventh Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first six embodiments, wherein a powder comprising afluoride of R³ and/or an oxyfluoride of R⁴ is used whereby fluorine isabsorbed in the magnet form along with R³ and/or R⁴.

Eighth Embodiment

A method for preparing a rare earth permanent magnet material accordingto the seventh embodiment, wherein in the powder comprising a fluorideof R³ and/or an oxyfluoride of R⁴, R³ and/or R⁴ contains at least 10atom % of Dy and/or Tb, and the total concentration of Nd and Pr in R³and/or R⁴ is lower than the total concentration of Nd and Pr in R¹.

Ninth Embodiment

A method for preparing a rare earth permanent magnet material accordingto the seventh or eighth embodiment, wherein in the powder comprising afluoride of R³ and/or an oxyfluoride of R⁴, the R³ fluoride and the R⁴oxyfluoride are contained in a total amount of at least 10% by weight,with the balance being one or more members selected from among acarbide, nitride, oxide, hydroxide and hydride of R⁵ wherein R⁵ is oneor more elements selected from among rare earth elements inclusive of Yand Sc.

Tenth Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first nine embodiments, further comprising, after the heattreatment, effecting aging treatment at lower than the temperature ofthe heat treatment.

Eleventh Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first ten embodiments, wherein said powder comprising oneor more members selected from an oxide of R², a fluoride of R³, and anoxyfluoride of R⁴ wherein R², R³ and R⁴ each are one or more elementsselected from among rare earth elements inclusive of Y and Sc and havingan average particle size of up to 100 μm is disposed in the surface ofthe magnet form as a slurry thereof dispersed in an aqueous or organicsolvent.

Twelfth Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first eleven embodiments, wherein the sintered magnet formis cleaned with at least one of alkalis, acids and organic solvents,disposing the powder on the surface of the magnet form, and theneffecting the heat treatment.

Thirteenth Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first eleven embodiments, wherein a surface layer of thesintered magnet form is removed by shot blasting, disposing the powderon the surface of the magnet form, and then effecting the heattreatment.

Fourteenth Embodiment

A method for preparing a rare earth permanent magnet material accordingto any of the first thirteen embodiments, wherein cleaning with at leastone of alkalis, acids and organic solvents, grinding, or plating orpainting is carried out as a final treatment after the heat treatment.

BENEFITS OF THE INVENTION

The present invention ensures that a high-performance permanent magnet,especially a compact or thin permanent magnet, having a high remanenceand a high coercive force is manufactured at a high productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a demagnetization curve (curve H1) of a magnetform M1 which is prepared by the invention and a demagnetization curve(curve K1) of a magnet form P1 which is prepared only by machining andheat treatment.

FIG. 2 includes (a) a reflection electron image under SEM, and (b) a Dycompositional image, (c) Nd compositional image, (d) F compositionalimage, and (e) O compositional image by EPMA of the magnet form M1 whichis prepared by the invention.

FIG. 3 is a graph showing a demagnetization curve (curve H2) of a magnetform M5 which is prepared by the invention and a demagnetization curve(curve K2) of a magnet form P4 which is prepared only by machining andheat treatment.

BEST MODE FOR CARRYING OUT THE INVENTION

Now the invention will be described in more detail.

The invention pertains to a method for preparing a R—Fe—B sinteredmagnet material having a high remanence and coercive force.

In the method for preparing a rare earth permanent magnet material, anoxide, fluoride or oxyfluoride of a rare earth element is disposed onthe surface of a sintered magnet form composed of R¹—Fe—B composition toeffect heat treatment.

The R—Fe—B sintered magnet form may be obtained from a mother alloy in aconventional way by coarse pulverization, fine pulverization, compactingand sintering.

As used herein, R and R¹ each are selected from among rare earthelements inclusive of Y and Sc. R is mainly used for the magnet formobtained, and R¹ is mainly used for the starting material.

The mother alloy contains R¹, Fe, and B. R¹ represents one or moreelements selected from among rare earth elements inclusive of Y and Sc,examples of which include Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,Er, Yb, and Lu. Preferably R¹ is mainly composed of Nd, Pr, and Dy. Therare earth elements inclusive of Y and Sc should preferably account for10 to 15 atom %, especially 12 to 15 atom % of the entire alloy. Morepreferably, R¹ should contain either one or both of Nd and Pr in anamount of at least 10 atom %, especially at least 50 atom %. Boronshould preferably account for 3 to 15 atom %, especially 4 to 8 atom %of the entire alloy. The alloy may further contain 0 to 11 atom %,especially 0.1 to 5 atom % of one or more elements selected from amongAl, Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag,Cd, Sn, Sb, Hf, Ta, and W. The balance consists of Fe and incidentalimpurities such as C, N and O. Iron should preferably account for atleast 50 atom %, especially at least 65 atom % of the entire alloy. Itis acceptable that Co substitutes for part of Fe, for example, 0 to 40atom %, especially 0 to 15 atom % of Fe.

The mother alloy is obtained by melting the starting metals or alloys invacuum or in an inert gas, preferably Ar atmosphere, and then pouring ina flat mold or book mold, or pouring as by strip casting. An alternativemethod, called binary alloys method, is also applicable wherein an alloywhose composition is approximate to the R₂Fe₁₄B compound, the primaryphase of the present alloy and an R-rich alloy serving as a liquid phaseaid at the sintering temperature are separately prepared, crushed,weighed and admixed together. It is noted that since the alloy whosecomposition is approximate to the primary phase composition is likely toleave α-Fe depending on the cooling rate during the casting or the alloycomposition, it is subjected to homogenizing treatment, if desired forthe purpose of increasing the amount of R₂Fe₁₄B compound phase. Thehomogenization is achievable by heat treatment in vacuum or in an Aratmosphere at 700 to 1,200° C. for at least 1 hour. For the R-rich alloyserving as a liquid phase aid, not only the casting method describedabove, but also the so-called melt quenching method are applicable.

Furthermore, in the pulverizing step to be described below, at least onemember selected from a carbide, nitride, oxide and hydroxide of R¹ or amixture or composite thereof can be admixed with the alloy powder in anamount of 0.005 to 5% by weight.

The alloy is generally coarsely pulverized to a size of 0.05 to 3 mm,especially 0.05 to 1.5 mm. For the coarse pulverizing step, a Brown millor hydriding pulverization is used, with the hydriding pulverizationbeing preferred for the alloy prepared by strip casting. The coarsepowder is then finely pulverized to a size of 0.2 to 30 μm, especially0.5 to 20 μm, for example, on a jet mill using high pressure nitrogen.

The fine powder is compacted in a magnetic field by a compressionmolding machine and introduced into a sintering furnace. The sinteringis carried out in vacuum or an inert gas atmosphere, typically at 900 to1,250° C., especially 1,000 to 1,100° C.

The sintered magnet thus obtained contains 60 to 99% by volume,preferably 80 to 98% by volume of the tetragonal R₂Fe₁₉B compound as theprimary phase, with the balance being 0.5 to 20% by volume of an R-richphase, 0 to 10% by volume of a B-rich phase, and at least one ofcarbides, nitrides, oxides and hydroxides resulting from incidentalimpurities or additives or a mixture or composite thereof.

The sintered block is machined into a preselected shape. Its size is notlimited. In the practice of the invention, the shape should preferablyhave a dimension of up to 100 mm, preferably up to 50 mm, especially upto 20 mm along its maximum side and a dimension of up to 10 mm,preferably up to 5 mm, especially up to 2 mm in a magnetic anisotropydirection, for the reason that the amount of R², R³ or R⁴ absorbed inthe magnet form from the powder comprising one or more members selectedfrom an oxide of R², a fluoride of R³, and an oxyfluoride of R⁴ disposedon the magnet surface becomes greater as the specific surface area ofthe magnet form is larger, that is, the dimensions thereof are smaller.More preferably, the dimension in a magnetic anisotropy direction shouldbe up to 10 mm, preferably up to 5 mm, especially up to 1 mm.

The shape may have a dimension of at least 0.1 mm along its maximum sideand a dimension of at least 0.05 mm in a magnetic anisotropy direction,although the invention is not limited thereto.

Disposed on the surface of the machined magnet form is a powdercomprising one or more members selected from an oxide of R², a fluorideof R³, and an oxyfluoride of R⁴. It is noted that R², R³ and R⁴ each areone or more elements selected from among rare earth elements inclusiveof Y and Sc and preferably contains at least 10 atom %, more preferablyat least 20 atom %, most preferably at least 40 atom % of Dy or Tb.

In this case, it is preferred from the object of the present inventionthat in the powder comprising a fluoride of R³ and/or an oxyfluoride ofR⁴, R³ and/or R⁴ contains at least 10 atom % of Dy and/or Tb, and thetotal concentration of Nd and Pr in R³ and/or R⁴ is lower than the totalconcentration of Nd and Pr in R¹.

For the reason that a more amount of R², R³ or R⁴ is absorbed as thefilling factor of the powder in the magnet surface-surrounding space ishigher, the filling factor should be at least 10% by volume, preferablyat least 40% by volume, calculated as an average value in the magnetsurrounding space from the magnet surface to a distance of 1 mm, inorder for the invention to attain its effect.

One exemplary technique of disposing or applying the powder is bydispersing a fine powder comprising one or more members selected from anoxide of R², a fluoride of R³, and an oxyfluoride of R⁴ in water or anorganic solvent to form a slurry, immersing the magnet form in theslurry, and drying in hot air or in vacuum or drying in the ambient air.Alternatively, the powder can be applied by spray coating or the like.Any such technique is characterized by ease of application and masstreatment.

The particle size of the fine powder affects the reactivity when the R²,R³ or R⁴ component in the powder is absorbed in the magnet. Smallerparticles offer a larger contact area that participates in the reaction.In order for the invention to attain its effect, the powder disposedaround the magnet should desirably have an average particle size of upto 100 μm, preferably up to 10 μm. Although the lower limit is notrestricted, it is preferably at least 1 nm. It is noted that the averageparticle size is determined as a weight average diameter D₅₀ (particlediameter at 50% by weight cumulative, or median diameter) uponmeasurement of particle size distribution by laser light diffractometry.

The oxide of R², fluoride of R³, and oxyfluoride of R⁴ used herein aretypically R² ₂O₃, R³F₃, and R⁴OF, respectively, although they generallyrefer to oxides containing R² and oxygen, fluorides containing R³ andfluorine, and oxyfluorides containing R⁴, oxygen and fluorine,additionally including R²O_(n), R³F_(n), and R⁴O_(m)F_(n) wherein m andn are arbitrary positive numbers, and modified forms in which part ofR², R³ or R⁴ is substituted or stabilized with another metal element aslong as they can achieve the benefits of the invention.

The powder disposed on the magnet surface contains the oxide of R²,fluoride of R³, oxyfluoride of R⁴ or a mixture thereof, and mayadditionally contain at least one member selected from among a carbide,nitride, hydroxide and hydride of R⁵ or a mixture or composite thereofwherein R⁵ is one or more elements selected from among rare earthelements inclusive of Y and Sc. In case of using a fluoride of R³ and/oran oxyfluoride of R⁴, an oxide of R⁵ may additionally be contained.Further, the powder may contain a fine powder of boron, boron nitride,silicon, carbon or the like, or an organic compound such as stearic acidin order to promote the dispersion or chemical/physical adsorption ofthe powder. In order for the invention to attain its effect efficiently,the powder should contain at least 10% by weight, preferably at least20% by weight of the oxide of R², fluoride of R³, oxyfluoride of R⁴ or amixture thereof. It is recommended that the oxide of R², fluoride of R³or oxyfluoride of R⁴ as the main component account for at least 50% byweight, more preferably at least 70% by weight, and even more preferablyat least 90% by weight based on the entire powder.

After the powder comprising the oxide of R², fluoride of R³, oxyfluorideof R⁴ or a mixture thereof is disposed on the magnet surface, the magnetand the powder are heat treated in vacuum or in an atmosphere of aninert gas such as argon (Ar) or helium (He). This treatment is referredto as absorption treatment, hereinafter. The temperature of absorptiontreatment is equal to or below the sintering temperature of the magnetform. The treatment temperature is limited for the following reason.

If treatment is done at a temperature above the sintering temperature(designated Ts in ° C.) of the relevant sintered magnet, there ariseproblems like (1) the sintered magnet alters its structure and fails toprovide excellent magnetic properties; (2) the sintered magnet fails tomaintain its dimensions as machined due to thermal deformation; and (3)the diffusing R can diffuse into the interior of magnet grains beyondthe grain boundaries in the magnet, resulting in a reduced remanence.The treatment temperature should thus be equal to or below the sinteringtemperature, and preferably equal to or below (Ts-10° C. The lower limitof temperature, which may be selected as appropriate, is typically atleast 350° C. The time of absorption treatment is from 1 minute to 100hours. The absorption treatment is not completed within less than 1minutes whereas more than 100 hours of treatment gives rise to theproblems that the sintered magnet alters its structure and theinevitable oxidation and evaporation of components adversely affect themagnetic properties. The more preferred time is 5 minutes to 8 hours,especially 10 minutes to 6 hours.

Through the absorption treatment described above, R², R³ or R⁴ which hasbeen contained in the powder disposed on the magnet surface istransferred and concentrated in the rare earth-rich grain boundary phasecomponent within the magnet where R2, R³ or R⁴ substitutes near thesurface layer of R₂Fe₁₄B primary phase grains. In the event the powdercontains the fluoride of R³ or oxyfluoride of R⁴, part of the fluorinecontained in the powder is absorbed in the magnet along with the R³ orR⁴, substantially facilitating the supply of R³ or R⁴ from the powderand the diffusion of R³ or R⁴ at grain boundaries in the magnet.

The rare earth element contained in the oxide of R², fluoride of R³, oroxyfluoride of R⁴ is one or more elements selected from rare earthelements inclusive of Y and Sc. Since the elements which are mosteffective in enhancing crystal magnetic anisotropy when concentrated atthe surface layer are dysprosium and terbium, it is preferred that Dyand Tb be contained in the powder in a total amount of at least 10 atom% based on the rare earth elements, with a total amount of at least 20atom % being more preferred. It is also preferred that the totalconcentration of Nd and Pr in R², R³ and R⁴ be lower than the totalconcentration of Nd and Pr in R¹.

As a result of the absorption treatment, the coercive force of theR—Fe—B sintered magnet is efficiently increased without entailing asubstantial loss of remanence.

The absorption treatment can be conducted by, for example, putting thesintered magnet form in a slurry obtained by dispersing the powder inwater or an organic solvent to dispose the powder on the surface of thesintered magnet form, and then effecting heat treatment. In theabsorption treatment, the magnets are covered with the powder so thatthe magnets are kept apart, preventing the magnets from being fusedtogether after the absorption treatment albeit high temperature.Additionally, the powder is not bonded to the magnets after the heattreatment. This permits a number of magnets to be placed in a containerfor heat treatment therein, indicating that the preparation method ofthe invention is also improved in productivity.

Also preferably, the absorption treatment is followed by agingtreatment. The aging treatment is desirably at a temperature which isbelow the absorption treatment temperature, preferably from 200° C. to atemperature lower than the absorption treatment temperature by 10° C.,and more preferably from 350° C. to a temperature lower than theabsorption treatment temperature by 10° C. The atmosphere is preferablyvacuum or an inert gas such as Ar or He. The time of aging treatment isfrom 1 minute to 10 hours, preferably from 10 minutes to 5 hours, andmore preferably from 30 minutes to 2 hours.

It is noted for the machining of the sintered magnet form beforedisposing the powder on the sintered magnet form that if the coolantused in the machining tool is aqueous, or if the surface being machinedis exposed to high temperature during the machining, there is alikelihood of an oxide film forming on the machined surface, which oxidefilm can inhibit the absorption reaction of R², R³ or R⁴ component fromthe powder to the magnet form. In such a case, the oxide film is removedby cleaning with at least one of alkalis, acids and organic solvents orby shot blasting before adequate absorption treatment is carried out.

Examples of the alkali used herein include potassium pyrophosphate,sodium pyrophosphate, potassium citrate, sodium citrate, potassiumacetate, sodium acetate, potassium oxalate, sodium oxalate, etc.Examples of the acid used herein include hydrochloric acid, nitric acid,sulfuric acid, acetic acid, citric acid, tartaric acid, etc. Examples ofthe organic solvent used herein include acetone, methanol, ethanol,isopropyl alcohol, etc. Herein, the alkali or acid may be used as anaqueous solution having an appropriate concentration not to attack themagnet form.

Further, a surface layer of the sintered magnet form may be removed byshot blasting before disposing the powder.

The magnet, which has been subjected to the absorption treatment andoptionally, subsequent aging treatment, may be again cleaned with atleast one of alkalis, acids and organic solvents or machined to apractical shape. Moreover, the process including absorption treatment,aging treatment, and cleaning or machining may further includesubsequent plating or painting.

The permanent magnet material thus obtained finds use as ahigh-performance, compact or thin permanent magnet having an increasedcoercive force.

EXAMPLE

Examples are given below for illustrating the present invention, but thescope of the invention is not limited thereby. In Examples, the fillingfactor of dysprosium oxide or dysprosium fluoride in the magnetsurface-surrounding space is calculated from a weight gain of the magnetafter powder treatment and the true density of powder material.

Example 1 and Comparative Examples 1-2

A thin plate of alloy was prepared by a so-called strip castingtechnique, specifically by weighing predetermined amounts of Nd, Co, Aland Fe metals having a purity of at least 99% by weight and ferroboron,induction heating in an argon atmosphere for melting, and casting thealloy melt on a copper single roll in an argon atmosphere. The resultingalloy had a composition of 13.5 atom % Nd, 1.0 atom % Co, 0.5 atom % Al,5.8 atom % B, and the balance of Fe and is designated Alloy A. Alloy Awas hydrided and then heated at 500° C. for partial dehydriding whileevacuating to vacuum. By this so-called hydriding pulverization, thealloy was pulverized into a coarse powder having a size of up to 30mesh. Another alloy was prepared by weighing predetermined amounts ofNd, Tb, Fe, Co, Al and Cu metals having a purity of at least 99% byweight and ferroboron, induction heating in an argon atmosphere formelting, and casting. The resulting alloy had a composition of 20 atom %Nd, 10 atom % Tb, 24 atom % of Fe, 6 atom % B, 1 atom % of Al, 2 atom %of Cu, and the balance of Co and is designated Alloy B. Using a Brownmill in a nitrogen atmosphere, Alloy B was coarsely pulverized to a sizeof up to 30 mesh.

Subsequently, Alloy A powder and Alloy B powder were weighed in amountsof 90% and 10% by weight, respectively, and mixed together on a Vblender which had been purged with nitrogen. On a jet mill usinghigh-pressure nitrogen gas, the mixed powder was finely pulverized to amass median particle diameter of 4 μm. The mixed fine powder wascompacted in a nitrogen atmosphere under a pressure of about 1 ton/cm²while being oriented in a magnetic field of 15 kOe. The compact was thenplaced in a sintering furnace in an argon atmosphere where it wassintered at 1,060° C. for 2 hours, obtaining a magnet block dimensioned10 mm×20 mm×15 mm (thick). Using a diamond cutter, the magnet block wasmachined on all the surfaces to 4 mm×4 mm×0.5 mm (magnetic anisotropydirection).

The machined magnet form was cleaned with an alkaline solution, cleanedwith acids and dried. Steps of rinsing with deionized water wereincluded before and after each cleaning step.

Subsequently, dysprosium fluoride having an average particle size of 5μm was mixed with ethanol at a weight fraction of 50%, in which themagnet form was immersed for one minute with ultrasonic waves beingapplied. The magnet form was pulled up and immediately dried with hotair. At this point, the filling factor of dysprosium fluoride in themagnet surface-surrounding space was 45%. The magnet form was subjectedto absorption treatment in an argon atmosphere at 900° C. for one hour,then to aging treatment at 500° C. for one hour, and quenched, obtaininga magnet form designated M1. For comparison purposes, a magnet formdesignated P1 was prepared by subjecting it to only heat treatment.

In FIG. 1, demagnetization curves of magnet forms M1 and P1 are depictedas curves H1 and K1, respectively, and their magnetic properties areshown in Table 1. The inventive magnet was found to offer an increase incoercive force of 500 kAm⁻¹ relative to the coercive force of the magnetP1 which had not been subjected to dysprosium absorption treatment. Adrop of remanence was 5 mT.

For comparison purposes, a magnet was prepared using an alloycomposition in which part of Nd in Alloy A was substituted with Dy. Thismagnet was designed to achieve an increase in coercive force of 500kAm⁻¹, but its remanence dropped by 50 mT. The magnetic properties ofthis magnet form P2 are also shown in Table 1.

FIG. 2 illustrates a reflection electron image under SEM, andcompositional images of Dy, Nd, F and O by EPMA of the magnet form M1.Since the magnet before the treatment does not contain Dy and F, thepresence of Dy and F in FIG. 2 is attributed to the absorption treatmentof the invention. Dysprosium absorbed is concentrated only in proximityto grain boundaries while fluorine (F) is also present at grainboundaries and bonds with oxides, which are contained as incidentalimpurities within the magnet before the treatment, to form oxyfluorides.This distribution of Dy enables to increase the coercive force whileminimizing a drop of remanence.

Example 2

By the same procedure as in Example 1, a magnet form of 20 mm×30 mm×3 mmwas prepared.

Dysprosium oxyfluoride having an average particle size of 10 μm wasmixed with ethanol at a weight fraction of 50%, in which the magnet formwas immersed for one minute with ultrasonic waves being applied. Themagnet form was pulled up and immediately dried with hot air. At thispoint, the filling factor of dysprosium oxyfluoride in the magnetsurface-surrounding space was 45%. The magnet form was subjected toabsorption treatment in an argon atmosphere at 900° C. for one hour,then to aging treatment at 500° C. for one hour, and quenched, obtaininga magnet form designated M2.

The magnetic properties of magnet form M2 are also shown in Table 1. Theinventive magnet was found to offer an increase in coercive force of 470kAm⁻¹ relative to the coercive force of the magnet P1 which had not beensubjected to dysprosium absorption treatment. A drop of remanence was 3mT.

Example 3

By the same procedure as in Example 1, a magnet form of 10 mm×20 mm×1.5mm was prepared.

Terbium fluoride having an average particle size of 5 μm was mixed withethanol at a weight fraction of 50%, in which the magnet form wasimmersed for one minute with ultrasonic waves being applied. The magnetform was pulled up and immediately dried with hot air. At this point,the filling factor of terbium fluoride in the magnet surface-surroundingspace was 45%. The magnet form was subjected to absorption treatment inan argon atmosphere at 900° C. for one hour, then to aging treatment at500° C. for one hour, and quenched, obtaining a magnet form designatedM3.

The magnetic properties of magnet form M3 are also shown in Table 1. Theinventive magnet was found to offer an increase in coercive force of 800kAm⁻¹ relative to the coercive force of the magnet P1 which had not beensubjected to terbium absorption treatment. A drop of remanence was 5 mT.

Example 4 and Comparative Example 3

A thin plate of alloy was prepared by a so-called strip castingtechnique, specifically by weighing predetermined amounts of Nd, Co, Al,Fe and Cu metals having a purity of at least 99% by weight andferroboron, induction heating in an argon atmosphere for melting, andcasting the alloy melt on a copper single roll in an argon atmosphere.The resulting alloy had a composition of 13.5 atom % Nd, 1.0 atom % Co,0.5 atom % Al, 0.2 atom % of Cu, 5.9 atom % B, and the balance of Fe.The alloy was hydrided and then heated to 500° C. for partialdehydriding while evacuating to vacuum. By this so-called hydridingpulverization, the alloy was pulverized into a coarse powder having asize of up to 30 mesh.

On a jet mill using high-pressure nitrogen gas, the coarse powder wasfinely pulverized to a mass median particle diameter of 4 μm. Theresulting fine powder was compacted in a nitrogen atmosphere under apressure of about 1 ton/cm² while being oriented in a magnetic field of15 kOe. The compact was then placed in a sintering furnace in an argonatmosphere where it was sintered at 1,060° C. for 2 hours, obtaining amagnet block dimensioned 10 mm×20 mm×15 mm (thick). Using a diamondcutter, the magnet block was machined on all the surfaces to 20 mm×4mm×1 mm.

The machined magnet form was cleaned with an alkaline solution, cleanedwith acids and dried. Steps of rinsing with deionized water wereincluded before and after each cleaning step.

Subsequently, terbium fluoride having an average particle size of 5 μmwas mixed with ethanol at a weight fraction of 50%, in which the magnetform was immersed for one minute with ultrasonic waves being applied.The magnet form was pulled up and immediately dried with hot air. Atthis point, the filling factor of terbium fluoride in the magnetsurface-surrounding space was 45%. The magnet form was subjected toabsorption treatment in an argon atmosphere at 900° C. for one hour,then to aging treatment at 500° C. for one hour, and quenched, obtaininga magnet form designated M4. For comparison purposes, a magnet formdesignated P3 was prepared by subjecting it to only heat treatment.

The magnetic properties of magnet forms M4 and P3 are also shown inTable 1. The inventive magnet was found to offer an increase in coerciveforce of 800 kAm⁻¹ relative to the coercive force of the magnet P3 whichhad not been subjected to terbium absorption treatment. A drop ofremanence was 5 mT.

TABLE 1 Br HcJ (BH)max (T) (kAm⁻¹) (kJm⁻³) Example 1 M1 1.415 1,500 390Example 2 M2 1.417 1,470 393 Example 3 M3 1.415 1,800 390 Example 4 M41.445 1,600 407 Comparative Example 1 P1 1.420 1,000 395 ComparativeExample 2 P2 1.370 1,500 368 Comparative Example 3 P3 1.450 800 412

Example 5 and Comparative Examples 4-5

A thin plate of alloy was prepared by a so-called strip castingtechnique, specifically by weighing predetermined amounts of Nd, Co, Aland Fe metals having a purity of at least 99% by weight and ferroboron,induction heating in an argon atmosphere for melting, and casting thealloy melt on a copper single roll in an argon atmosphere. The resultingalloy had a composition of 13.5 atom % Nd, 1.0 atom % Co, 0.5 atom % Al,5.8 atom % B, and the balance of Fe, and is designated Alloy C. Alloy Cwas hydrided and then heated at 500° C. for partial dehydriding whileevacuating to vacuum. By this so-called hydriding pulverization, thealloy was pulverized into a coarse powder having a size of up to 30mesh. Another alloy was prepared by weighing predetermined amounts ofNd, Tb, Fe, Co, Al and Cu metals having a purity of at least 99% byweight and ferroboron, induction heating in an argon atmosphere formelting, and casting. The resulting alloy had a composition of 20 atom %Nd, 10 atom % Tb, 24 atom % of Fe, 6 atom % B, 1 atom % of Al, 2 atom %of Cu, and the balance of Co, and is designated Alloy D. Using a Brownmill in a nitrogen atmosphere, Alloy D was coarsely pulverized to a sizeof up to 30 mesh.

Subsequently, Alloy C powder and Alloy D powder were weighed in amountsof 90% and 10% by weight, respectively, and mixed together on a Vblender which had been purged with nitrogen. On a jet mill usinghigh-pressure nitrogen gas, the mixed powder was finely pulverized to amass median particle diameter of 4 μm. The mixed fine powder wascompacted in a nitrogen atmosphere under a pressure of about 1 ton/cm²while being oriented in a magnetic field of 15 kOe. The compact was thenplaced in a sintering furnace in an argon atmosphere where it wassintered at 1,060° C. for 2 hours, obtaining a magnet block dimensioned10 mm×20 mm×15 mm (thick). Using a diamond cutter, the magnet block wasmachined on all the surfaces to 4 mm×4 mm×0.5 mm (magnetic anisotropydirection).

The machined magnet form was cleaned with an alkaline solution, cleanedwith acids and dried. Steps of rinsing with deionized water wereincluded before and after each cleaning step.

Subsequently, dysprosium oxide having an average particle size of 1 μmwas mixed with ethanol at a weight fraction of 50%, in which the magnetform was immersed for one minute with ultrasonic waves being applied.The magnet form was pulled up and immediately dried with hot air. Atthis point, the filling factor of dysprosium oxide in the magnetsurface-surrounding space was 50%. The magnet form was subjected toabsorption treatment in an argon atmosphere at 900° C. for one hour,then to aging treatment at 500° C. for one hour, and quenched, obtaininga magnet form designated M5. For comparison purposes, a magnet formdesignated P4 was prepared by subjecting it to only heat treatment.

In FIG. 3, demagnetization curves of magnet forms M5 and P4 are depictedas curves H2 and K2, respectively, and their magnetic properties areshown in Table 2. The inventive magnet was found to offer an increase incoercive force of 400 kAm⁻¹ relative to the coercive force of the magnetP4 which had not been subjected to dysprosium absorption treatment. Nodrop of remanence was found.

In Comparative Example 5, a magnet was prepared as in Example 5 asidefrom using an alloy composition in which part of Nd in Alloy C wassubstituted with Dy and omitting the absorption treatment. This magnetwas designed to achieve an increase in coercive force of 400 kAm⁻¹, butits remanence dropped by 40 mT. The magnetic properties of this magnetform P5 are also shown in Table 2.

TABLE 2 Br HcJ (BH)max (T) (kAm⁻¹) (kJm⁻³) Example 5 M5 1.420 1,400 395Comparative Example 4 P4 1.420 1,000 395 Comparative Example 5 P5 1.3801,400 375

1. A method for preparing a rare earth permanent magnet materialcomprising the steps of: disposing a powder comprising one or moremembers selected from the group consisting of an oxide of R², a fluorideof R³, and an oxyfluoride of R⁴ wherein R², R³ and R⁴ each are one ormore elements selected from the group consisting of rare earth elementsinclusive of Y and Sc on a sintered magnet form to be heat treated of aR¹—Fe—B composition, wherein R¹ is one or more elements selected fromamong rare earth elements inclusive of Y and Sc, the sintered magnetform having a dimension of at least 0.5 mm in a magnetic anisotropydirection and at least 4 mm along its maximum side, and then heattreating the magnet form and the powder at a temperature equal to orbelow the sintering temperature of the magnet in vacuum or in an inertgas.
 2. The method for preparing a rare earth permanent magnet materialaccording to claim 1, wherein the sintered magnet form to be heattreated has a shape having a dimension of up to 100 mm along its maximumside and a dimension of up to 10 mm in a magnetic anisotropy direction.3. The method for preparing a rare earth permanent magnet materialaccording to claim 2, wherein the sintered magnet form to be heattreated has a shape having a dimension of up to 20 mm along its maximumside and a dimension of up to 2 mm in a magnetic anisotropy direction.4. The method for preparing a rare earth permanent magnet materialaccording to claim 1, wherein the powder comprising one or more membersselected from the group consisting of an oxide of R², a fluoride of R³,and an oxyfluoride of R⁴ is present in a magnet-surrounding space withina distance of 1 mm from the surface of the magnet form and at an averagefilling factor of at least 10%.
 5. The method for preparing a rare earthpermanent magnet material according to claim 1, wherein the powdercomprising one or more members selected from the group consisting of anoxide of R², a fluoride of R³, and an oxyfluoride of R⁴ has an averageparticle size of up to 100 μm.
 6. The method for preparing a rare earthpermanent magnet material according to claim 1, wherein R2, R³ or R⁴contains at least 10 atom % of Dy and/or Tb.
 7. The method for preparinga rare earth permanent magnet material according to claim 1, wherein apowder comprising a fluoride of R³ and/or an oxyfluoride of R⁴ is usedwhereby fluorine is absorbed in the magnet form along with R³ and/or R⁴.8. The method for preparing a rare earth permanent magnet materialaccording to claim 7, wherein R³ and/or R⁴ contains at least 10 atom %of Dy and/or Tb, and the total concentration of Nd and Pr in R³ and/orR⁴ is lower than the total concentration of Nd and Pr in R¹.
 9. Themethod for preparing a rare earth permanent magnet material according toclaim 7, wherein in the powder comprising a fluoride of R³ and/or anoxyfluoride of R⁴, the R³ fluoride and the R⁴ oxyfluoride are containedin a total amount of at least 10% by weight, with the balance being oneor more members selected from among a carbide, nitride, oxide, hydroxideand hydride of R⁵ wherein R⁵ is one or more elements selected from thegroup consisting of rare earth elements inclusive of Y and Sc.
 10. Themethod for preparing a rare earth permanent magnet material according toclaim 1, further comprising, after said heat treatment, effecting agingtreatment at a temperature from 350° C. to a temperature lower than thetemperature of the heat treatment.
 11. The method for preparing a rareearth permanent magnet material according to claim 1, wherein the powderis disposed in the surface of the magnet form as a slurry thereofdispersed in an aqueous or organic solvent.
 12. The method for preparinga rare earth permanent magnet material according to claim 1, wherein thesintered magnet form is cleaned with at least one of alkalis, acids andorganic solvents before said step of disposing the powder on the surfaceof the magnet form.
 13. The method for preparing a rare earth permanentmagnet material according to claim 1, wherein a surface layer of thesintered magnet form is removed by shot blasting before said step ofdisposing the powder on the surface of the magnet form.
 14. The methodfor preparing a rare earth permanent magnet material according to claim1, wherein cleaning with at least one of alkalis, acids and organicsolvents, grinding, plating, or painting is carried out as a finaltreatment after said heat treatment.
 15. The method for preparing a rareearth permanent magnet material according to claim 1, wherein thesintered magnet has a dimension of 4 to 100 mm along its maximum side.16. The method for preparing a rare earth permanent magnet materialaccording to claim 1, wherein the sintered magnet form to be heattreated has a shape having a dimension of 0.5 to 10 mm in a magneticanisotropy direction.
 17. The method for preparing a rare earthpermanent magnet material according to claim 1, wherein the sinteredmagnet form to be heat treated is obtained by compacting and sinteringpowder of a mother alloy containing R¹, Fe, and B, and machining thethus-obtained sintered block to a shape having a dimension of 4 to 100mm along its maximum side and a dimension of up to 10 mm in a magneticanisotropy direction.